Structural frame for a solid polymer electrolyte electrochemical cell

ABSTRACT

A structural frame adapted for use in a solid polymer electrochemical cell, which comprises a generally planar organic plastic member having a plurality of horizontally and vertically spaced-apart shoulders protruding outwardly from opposing generally coplanar anolyte and catholyte surfaces of the plastic member. Electrically conductive, hydraulically impermeable, anolyte and catholyte corrosion resistant covers are matingly affixed to the respective surfaces of the plastic member. A hydraulically permeable, electrically conductive, current collector is contacting and positioned adjacent the catholyte cover. An ion exchange membrane, having an electrically conductive, electrocatalytic material bonded to or embedded in the surface of the membrane is positioned adjacent and in contact with the current collector. The frames are removably and sealably positioned in coplanar relationship, each plastic member being spaced apart by an anode on one side and a cathode on the other.

BACKGROUND OF THE INVENTION

This invention relates to an improvement in the structure of filterpress solid polymer electrolyte electrolysis cells. More particularly itrelates to those of such cells which employ permselective ion exchangemembranes having an electrocatalytic material bonded to or embedded inthe membrane and acting as anodes or cathodes. Such cells areparticularly useful in the electrolysis of aqueous solutions of alkalimetal chlorides; especially in the electrolysis of aqueous solutions ofsodium chloride (sodium chloride brine). The cell structure may also beused in electrolyzing other solutions to make products such as potassiumhydroxide, iodine, bromine, bromic acid, persulfuric acid, chloric acid,adiponitrile and other organic compounds made by electrolysis.

It is well established that various chemicals can be produced in anelectrolytic cell containing an anode and a cathode. For example, alkalimetal chlorates, such as sodium chlorate, have been formedelectrolytically from a sodium chloride brine in cells without aseparator positioned between the anode and the cathode.

When a separator, such as a liquid permeable asbestos orpolytetrafluoroethylene diaphragm or a substantially completely liquidimpervious ion exchange membrane, is used in a cell to electrolyze asodium chloride brine, the electrolytic products will normally begaseous chlorine, hydrogen gas, and an aqueous solution containingsodium hydroxide.

For a number of years gaseous chlorine was produced in electrolyticcells wherein an asbestos diaphragm was interposed between finger-like,anodes and cathodes which were interleaved together. During the pastseveral years it has become apparent that the use of a substantiallyliquid impermeable cation exchange membrane may be preferable to themore well established diaphragm in instances where a higher purity, forexample a lower sodium chloride content, higher sodium hydroxide productis desired. It was found to be more convenient to fabricate ion exchangetype electrolytic cells from relatively flat or planar sheets of ionexchange membrane rather than to interleave the membrane between theanode and cathode within the older finger-like cells used with asbestosdiaphragms.

The newer, so-called flat plate electrolytic cells using a planar pieceof ion exchange membrane to separate the anolyte from catholytecompartments also have a plurality of solid, liquid impervious framesadapted to support the anode on one side and the cathode on the oppositeside. These frames have previously been constructed of materials such asmetal and plastic, but neither of these materials has been found to beentirely satisfactory. In any electrolytic cell, including bothmonopolar and bipolar cells, there is a possibility that electrolyte mayleak from within the cell to the exterior. In instances where suchleakage has occurred in cells with iron or other ferrous type frames, itwas found that the iron frame corroded or was itself electrolyticallyattacked. Plastic frames are not generally subject to the electrolyticattack, but are normally not resistant to the anolyte and/or catholytewithin the cell under operating conditions for extended periods of time,for example, several years.

Solid polymer electrolyte membranes consist of an ion exchange membranehaving an electrically conductive, electrocatalytic material embedded inor bonded to at least one side of the ion exchange membrane. Suchelectrodes are well known in the art and are illustrated in, forexample, U.S. Pat. Nos. 4,457,815 and 4,457,823. Solid polymerelectrolyte membranes have been used as electrodes in processes anddevices for the generation of chlorine and hydrogen by electrolysis ofan aqueous alkali metal halide and for the electrolysis of water. Thecatalytic electrodes at which the chlorine and hydrogen are produced arethin, porous, gas permeable catalytic electrodes which are bonded to orembedded in opposite surfaces of the membrane so that the chlorine andhydrogen are generated (substantially) at the electrode membraneinterfaces. This results in electrodes which have very low overvoltagesfor chlorine and hydrogen discharge.

It is desired to provide a solid polymer electrolyte electrolytic cellhaving a structural frame which would minimize the corrosion problemsand would increase the relatively short useful life attendant with thoseframes used by the prior art.

SUMMMARY OF THE INVENTION

The present invention is a structural frame adapted for use in a solidpolymer electrolytic cell which comprises a generally planar organicplastic member having a plurality of horizontally and verticallyspaced-apart shoulders protruding outwardly from opposing generallycoplanar anolyte and catholyte surfaces of the plastic member. Each ofthe shoulders annularly encircles and supports an electricallyconductive insert extending from an exterior face of a shoulder on thecatholyte surface of the plastic member, through the plastic member, toan exterior face of a shoulder on the anolyte surface of the plasticmember.

An electrically conductive, substantially completely hydraulicallyimpermeable anolyte cover is matingly affixed to the anolyte surface ofthe plastic member and adapted to minimize contact between the anolyteand the plastic member. The anolyte cover is resistant to the corrosiveeffects of the anolyte. An electrically conductive, substantiallycompletely hydraulically impermeable catholyte cover is matingly affixedto the catholyte surface of the plastic member and adapted to minimizecontact between the catholyte and the plastic member. The catholytecover is a metal resistant to the corrosive effects of the catholyte.Both the anolyte cover and the catholyte cover may be made from a metal,or, optionally made from another material and have metallic insertsmolded in at the points where they contact the metallic inserts whichpass through the plastic member.

A hydraulically permeable, electrically conductive, current collector iscontacting and positioned adjacent to the catholyte cover. A ionexchange membrane, having an electrically conductive, electrocatalyticmaterial bonded to or embedded in the surface of the membrane, contactsand is positioned adjacent to the current collector.

The invention further includes an electrolytic cell utilizing aplurality of the above described structural frames removably andsealably positioned in a generally coplanar relationship with each otherand with each of the plastic members being spaced apart at least by ananode on one side of the plastic member and a cathode on an opposingside of the plastic member.

DESCRIPTION OF THE DRAWINGS

The FIGURE is an exploded sectional side view of the cell structure of asolid polymer electrolyte filter press-type cell unit 10 which employsthe unitary central cell element 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE shows structural frame 10 which achieves the above objects.It is illustrated for use in an electrolytic cell for producing gaseouschlorine from an aqueous alkali metal hydroxide solution. Although thepresent invention can be beneficially employed to produce chlorine andvarious alkali metal hydroxide solutions, it is preferred to use sodiumchloride as the primary salt in the starting brine since this particularsalt is readily available commercially and there are many wellestablished uses for sodium hydroxide produced electrolytically.

The cell structure 10 includes a generally planar organic plastic member12 which can be produced by commercial and known procedures into a shapewith a plurality of horizontally and vertically spaced apart shoulders14, 14a, 14b, and 14c. The shoulders 14, 14a, 14b, and 14c protrudeoutwardly from catholyte and anolyte surfaces 18 and 16, respectively.The peripheral surfaces 20, 20a, 20b, and 20c of the plastic member 12define the outer surface of the electrolytic cell 10 when a plurality ofthe plastic members are positioned together. The peripheralconfiguration of the plastic members 12 is optional and can be varied tosuit the particular configuration of the electrolytic cell shapedesired.

The number, size and shape of these shoulders may be an importantconsideration in both the design and operation of the present invention.They may be square, rectangular, conical, cylindrical, or any otherconvenient shape when viewed in sections taken either parallel orperpendicular to the central portion. The shoulders may have anelongated shape to form a series of spaced ribs distributed over thesurface of the plastic member.

A number of plastic materials are suitable for use in the presentinvention for the construction of plastic member 12. Without intendingto be limited by the specific organic materials hereinafter delineated,examples of such suitable materials include polyethylene; polypropylene;polyvinylchloride; chlorinated polyvinyl chloride; acrylonitrile,polystyrene, polysulfone, styrene acrylonitrile, butadiene and styrenecopolymers; epoxy; vinyl esters; polyesters; and fluoroplastics andcopolymers thereof. It is preferred that a material such aspolypropylene be used for the structural member 12 since it produces ashape with adequate structural integrity at elevated temperatures, isreadily available, and is relatively inexpensive with respect to othersuitable materials.

It is surprising that the plastic member 12 can be produced by any of anumber of processes known well to those skilled in the art of plasticmolding. Such molding processes include, for example, injection molding,compression molding, transfer molding, and casting. Of these processes,injection molding has been found to satisfactorily produce a structurewith adequate strength for use in an electrolytic cell. Preferably, theplastic is injected into a mold containing the desired number of inserts(discussed later). In this manner, the plastic member is a one-piecemember which fits tightly around the inserts, holds them in place, andprovides a high degree of support to them. Such a configurationminimizes the likelihood that the inserts will separate from the plasticmember and become loose. The ease of molding relatively complex shapesand the strength of the finished injection molded article contribute tomaking this process preferred for making the herein described structuralmember. This is a considerable advantage over the prior art where theplastic frame was molded first and then the electrical conductors weresubsequently installed.

When the plastic member 12 is employed in an electrolytic cell forproducing chlorine, the temperature of the cell and the plastic memberwill frequently reach, or be maintained at, temperatures of from about60° to about 90° Celsius. At these temperatures plastics, as do mostmaterials, expand a measurable amount. Any expansion and latercontraction on cooling of the plastic frame could result in electrolyteseeping from within the plurality of cells when joined together or, moreimportantly, could result in distortion of the anode and cathode whichare made of metallic expanded mesh or perforated sheets. Furthermore,the differential expansion between the plastic frame 22 and thecatholyte cover 22 and anolyte cover 24 would create stress on the weldswhich affix these covers to the inserts which are themselves molded inthe plastic frame.

To reduce, and preferably minimize, the difference in expansion betweenthe covers 22 and 24 and the plastic member 12, it is preferred toincorporate an additive to reduce thermally induced expansion of theplastic member. More preferably, the additive will also increase thestructural strength of the finished plastic article. Such additive canbe, for example, fiberglass, graphite fibers, carbon fibers, talc, glassbeads, pulverized mica, asbestos, and the like, and combinationsthereof. It is preferred that the plastic contain from about 5 to about75 weight percent additive, and more preferably from about 10 to about40 weight percent of the additive. Glass fibers can be readily mixedwith polypropylene to produce an injectable material suitable for use inthe present invention which results in a solid, physically strong bodywith a coefficient of expansion less than polypropylene not containingglass fibers.

It has been determined that the use of commercially availablepolypropylene which has been specially formulated to afford bonding withthe glass works particularly well. This results in a composite having alower coefficient expansion than a mixture of polypropylene and glassfibers. Such chemically-combined glass fiber reinforced polypropylene isavailable from, for example, Hercules, Inc., Wilmington, Del., asPro-fax PC072 polypropylene.

At least one electric conducting element, such as insert 26 or 28, ispositioned and preferably molded into the plastic member 12. The insert26 or 28 extends through the plastic member from the catholyte surface16 to the anolyte surface 18. The inserts 26 and are preferably retainedwithin the plastic member 12 by means of friction between the plasticand the insert. It is more preferable to increase the friction betweenthese two bodies by having an additional means to restrain the insertwithin the plastic. Such additional means include, for example grooves(one or more) around the circumference of the insert(s), keys welded tothe insert, hole(s) extending into and/or through the insert, slots,rings, collars, studs, or bosses.

The inserts 26 or 28 can be any material which will permit the flow ofan electric current between the catholyte cover 22 and the anolyte cover24. Since the covers 22 and 24 are preferably metallic, it is convenientto fabricate the insert from a metal, such as aluminum, copper, iron,steel, nickel, titanium, and the like, or alloys or physicalcombinations including such metals.

The shoulders and inserts should be spaced so they provide a somewhatuniform and low electrical potential gradient across the face of theelectrode to which they are attached. They should be spaced so that theyallow free fluid circulation from any unoccupied point within theirrespective electrolyte compartment to any other unoccupied point withinthat compartment. Thus the shoulders will be somewhat uniformly spacedapart from one another in their respective compartments.

To improve the flow of DC electric current between the covers 22 and 24,the inserts 26 or 28 is preferably made of a material weldablycompatible with the particular cover it contacts. For example, theinserts 26 or 28 may be a welded assembly of a steel insert with avanadium disk 262 or 262a interposed between, and welded to, both theinsert and the cover 24.

To prevent catholyte from contacting the plastic member 12 within theelectrolytic cell and causing deterioration of the plastic and/orleakage of electrolyte between the plastic and the inserts 26 or 28 fromcathode compartment 30 to anode compartment 32 the cover 22 is matinglycontacted with the catholyte surface 16 and the anolyte cover 24 ismatingly contacted with the anolyte surface 18. As is shown in theFIGURE, both the anolyte and the catholyte covers are so shaped tocorrespond and abut closely to the exterior surface of the plasticmember 12. It is important that the portions of both of the covers 22and 24 which are exposed to the anolyte or catholyte and span theplastic member contain no openings through which electrolyte orelectrolytic products can pass during operation of the electrolyticcell. The freedom from openings through the covers minimizes thelikelihood that electrolyte will leak or seep through holes or spacesaround gaskets of other seals and come into contact with the plasticmember. The degree of correspondence may be more or less thanillustrated in the FIGURE. In some instances, the electrode compartmentcovers 22 or 24 may about the frame 10 in one or more locations.

The anolyte cover 24 is made of a material which is resistant to theanolyte during operation of the cell. Normally, this material is notelectrolytically active, but the invention is still operable if thematerial does become or is active electrolytically. Suitable materialsfor the anolyte cover 24 are, for example, titanium, tantalum,zirconium, tungsten, and other valve metals not materially affected bythe anolyte. Titanium is preferred as the anolyte cover material.

The catholyte cover 22 is resistant to attack by the catholyte under theconditions present in the electrolytic cell. Suitable materials for thecatholyte cover include, for example, iron, steel, stainless steel,nickel, lead, molybdenum, and cobalt and alloys, including majorportions of these metals. Nickel, including nickel base alloys, ispreferably used for the catholyte cover material, since nickel andnickel alloys are generally resistant to the corrosive effects of thecatholyte, especially an aqueous catholyte solution containing up to atleast about 35 weight percent sodium hydroxide. Steel has also beenfound to be suitable, and relatively inexpensive, for use in a cell as acatholyte cover in the presence of a dilute (i.e., up to about 22 weightpercent) aqueous solution of sodium hydroxide.

To assist in assembling a plurality of the structural frames 10 into anelectrolytic cell it is desirable, although not essential, to haveflanges 34 and 34a extending outwardly from the main structural portionof the plastic member 12 along the periphery of such member. In apreferred embodiment the flanges extend outwardly from the plasticmember about the same distance as the inserts 26 or 28. Alternatively,but not preferred, separate spacer elements (not shown) could beutilized to build up the plastic member 12 sufficiently to permit anumber of the plastic members to be combined into a cell series withouthaving electrolyte, either anolyte or catholyte, leak from within thecatholyte and anolyte compartments 30 and 32, respectively, to anexterior portion of the cell.

The FIGURE shows the anode as being a hydraulically permeable metallicsheet, while the cathode 38 is illustrated as a solid polymerelectrolyte electrode. However, both electrodes could be a solid polymerelectrolyte electrode, or the anode could be a solid polymer electrolyteanode while the cathode is a hydraulically permeable metallic sheet.

The FIGURE further shows an anode 36, which is positively charged duringoperation of the cell from an external power source (not shown),electrically connected to the anolyte cover 24. Such electricalconnection is readily achieved by welding the anode 36 to the anodecover 24 where the anode cover comes into physical contact with theinserts 26 or 28. For improved electrical contact, the anolyte cover 24is welded to the inserts 26 or 28 and the anode 36 is welded to theanolyte cover 24 adjacent to the inserts 26 or 28.

Various means of welding can be utilized in the present invention, butit has been found highly satisfactory to use resistance or capacitancedischarge welding techniques. Other suitable welding techniques includetungsten inert gas welding (TIG) and metal inert gas (MIG) welding. Thiswelding serves a primary purpose of retaining the anode 36 in positionand not for electrical flow, although electric current will naturallypass through the welded areas.

At least one of the electrodes used in the cell of the present inventionconsists of a plurality of particles embedded in or bonded to an ionexchange membrane. Such electrodes are commonly referred to as solidpolymer electrodes and are well known in the art.

Cation exchange membranes are well known to contain fixed anionic groupsthat permit intrusion and exchange of cations, while almost totallyexcluding anions from passage therethrough. Generally the membrane has amatrix of a cross-linked polymer, to which are attached charged radicalssuch as --SO₃ (-1), --COO(-1), --PO₃ (-2), HPO₂ (-1), --AsO₃ (-2), andSeO₃ (-1). Vinyl addition polymers and condensation polymers may beemployed. The polymer can be, for example, styrene, divinyl benzene,polyethylene and fluorocarbons. Condensation polymers are, for example,phenol sulfuric acid, and formaldehyde resins. Representative of thetypes of permselective membranes envisioned for use with this inventionare those disclosed in the following U.S. Pat. Nos. 3,909,378;4,025,405; 4,065,366; 4,116,888; 4,123,336; 4,126,588; 4,151,052;4,176,215; 4,178,218; 4,192,725; 4,209,635; 4,212,713; 4,251,333;4,270,996; 4,329,435; 4,330,654; 4,337,137; 4,337,211; 4,340,680;4,357,218; 4,358,412; and 4,358,545. These patents are herebyincorporated by reference for the purpose of the membranes theydisclose.

In forming the solid polymer electrodes of the present invention, theelectrocatalytic materials may be bonded to and embedded in the ionexchange membrane, for example, in a thermoplastic fluorocarbon, or hotpressed to a thermoplastic fluorocarbon, or sintered, for example withpolytetrafluoroethylene. The electrocatalyst may be blended with othermaterials, such as graphite or silver for enhanced electricalconductivity. The membrane may itself have a roughened surface, asprovided by abrasion, a leachable pore forming material, or a volatilepore forming material. See for example U.S. Pat. No. 4,457,815.

Preferably, the electrocatalytic particles comprising the electrode(cathode or anode) are as fine a powder as is practical for use. Mostpreferably, the size of the particles are such that they pass through a400 mesh U.S.S. Standard nylon screen.

Preferably, the surface area of the particles, as observed by the BETnitrogen absorption method, have a surface area of at least about 25square meters per gram of electrocatalytic particles. More preferably,they have a surface area of from about 50 to about 150 square meters pergram of electrocatalytic particles. See for example U.S. Pat. No.4,457,823.

Exemplary electrocatalyst materials for use in forming the anodes ofsolid polymer electrodes for use in the cell of the present inventioninclude the platinum group metals as well as oxides and oxycompoundsthereof, for example, platinum black, and isostructural oxycompoundssuch as platinum group metal perovskites, platinum group metal spinelsand platinum group metal crystal defect semiconductors. An exemplarycrystal defect semiconductor is the isostructural rutheniumdioxide-titanium dioxide. By oxycompounds of the platinum group metalsare meant compositions of the platinum group with oxygen and anothermetal, as in spinels, perovskites, delafosites, and semiconductiveoxides.

Exemplary electrocatalyst materials for use in forming the cathodes ofsolid polymer electrodes of the present invention include the transitionmetals of group 8 of the Periodic Table, as exemplified by iron, cobaltand nickel, especially when present in forms having enhanced surfacearea, i.e., enhanced surface activity. The high surface area formsinclude leached codeposits of the transition metal with zinc, leacheddeposited Raney alloys, and blacks, for example, platinum black,palladium black and the like.

When the cathode is a solid polymer electrolyte cathode, optionally, theanode 36 may be a hydraulically permeable metal sheet formed frommaterials such as one of the common film-forming metals, which isresistant to the corrosive effects of the anolyte during the operationof the cell. The anode can be made permeable by several means including,for example, using a punched sheet or plate, an expanded mesh, or wovenwire. The anode should be sufficiently porous to permit anolyte andchlorine to pass therethrough. Suitable metals are well known to includetantalum, tungsten, columbium, zirconium, molybdenum, and preferably,titanium and alloys containing major amounts of these metals, coatedwith an activating substance, for example, an oxide of a platinum groupmetals, such as ruthenium, iridium, rhodium, platinum, palladium, eitheralone or in combination with an oxide of a film-forming metal. Othersuitable activating oxides include cobalt oxide.

The electrolytic cell of the present invention, when stacked adjacent toother such cells, may have the anode 36 and the cathode 38 spaced apartby an ion exchange membrane 44 which is in contact with the anode 36. Ifdesired, however, although not preferred, the membrane 44 could be incontact with the cathode 38 or be suspended between the two electrodes.It is important, that the ion exchange membrane 44 separate the anodecompartment 32 from the cathode compartment

The solid polymer electrolyte cathode 38 is electrically contacted withthe catholyte cover 22 through a heavy current collector 48, a mattress42, and a light current collector 46.

The heavy current collector 48 is preferably attached to the catholytecover 22 by welding, although it can be merely pressed against thecatholyte cover 22, bolted to it, or attached in some other way.

Suitable materials for use as the heavy current collector 48 may includeiron, nickel, lead, molybdenum, cobalt, and alloys including majoramounts of these metals. The heavy current collector should besufficiently porous to permit catholyte and hydrogen to passtherethrough. The heavy current collector can be made permeable byseveral means including, for example, using a punched sheet or plate, anexpanded mesh, or woven wire. The thickness of the material should besuch that it provides adequate support of a mattress 42 and a lightcurrent collector 46. Current collectors, are illustrated in, forexample, U.S. Pat. No. 4,444,641. That patent is hereby incorporated byreference for the purposes of its teaching about current collectors.

The mattress 42 may be constructed of metals which include iron, nickel,molybdenum, cobalt, and alloy including major amounts of these metals.Actual selection may depend upon the concentration of the alkali metalhydroxide in the aqueous solution. The mattress may be madehydraulically permeable, for example, using a woven and/or non-wovenwire construction which allows for compression of the material when"sandwiched" between heavy current collector 48 and light currentcollector 46. Various mattress suitable for use in the present inventionhave configurations which are illustrated in U.S. Pat. No. 4,340,452 andU.S. Pat. No. 4,444,632. These patents are incorporated by reference forthe purposes of the resilient cell elements that they teach.

Of course, it is within the scope of this invention for the electrolysiscell formed between the two cell segments to be a multi-compartmentelectrolysis cell using more than one solid polymer electrolytemembrane, e.g., a three-compartment cell with one membrane and one solidpolymer electrolyte membrane spaced from one another so as to form acompartment between them as well as the compartment formed on theopposite side of each between the separator and its respective adjacentfilter press cell unit.

To minimize leakage of electrolyte from the cell after assembling anumber of the structural frames 10 together, at least one gasket (notshown) is positioned between adjacent plastic frames. During assembly ofthe frames a compressive force is applied to the extremes of the framesto compress the gasket material so that it both seals the ion exchangemembrane 44, positions the membrane, and minimizes leakage ofelectrolyte from within the final cell series to the exterior of thecells. Preferably, the membrane 44 is positioned to substantiallyentirely prevent leakage of electrolyte from within the final cellseries to the exterior of the cells. Various gaskets materials can beused including, for example, fluorocarbon, chlorinated polyethylenerubber, and ethylene propylene diene terpolymer rubber. This inventionalso encompasses the use of gaskets on both sides of solid polymerelectrolyte membrane 44.

Inlets and outlets are provided through the flanges 34 or 34a to providepathways for the introduction of reactants into the cell and the removalof products from the cell. Such outlets may be positioned as desired andhave the number and cross-sectional area as determined by the operatingconditions of the cell. Such designs are well known in the art.

In operating the cell series as an electrolysis cell series for NaClbrine, certain operating conditions are preferred. In the anodecompartment a pH of from about 0.5 to about 5.0 is desired to bemaintained. The feed brine preferably contains only minor amounts ofmultivalent cations (less than about 80 parts per billion when expressedas calcium). More multivalent cation concentration is tolerated with thesame beneficial results if the feed brine contains carbon dioxide inconcentrations lower than about 70 ppm when the pH of the feed brine islower than 3.5.

Operating temperatures can range from about 0° to about 110° C., butpreferably from about 60° C. to about 80° C. Brine purified frommultivalent cations by ion-exchange resins after conventional brinetreatment has occurred is particularly useful in prolonging the life ofthe solid polymer electrolyte membrane. A low iron content in the feedbrine is desired to prolong the life of the solid polymer electrolytemembrane. Preferably the pH of the brine feed is maintained at a pHbelow 4.0 by the addition of hydrochloric acid.

Preferably the operating pressure is maintained at less than 7atmospheres.

Usually the cell is operated at a current density of from about 1.0 toabout 4.0 amperes per square inch, but in some cases operating above 4.0amps/in.² is quite acceptable.

We claim:
 1. A structural frame adapted for use in an electrolytic cellcomprising:an organic plastic member with a plurality of horizontallyand vertically spaced-apart shoulders protruding outwardly from opposinggenerally coplanar first and second surfaces of said plastic member; atleast one electrically conductive insert extending from an exterior faceof a shoulder on the first surface of the plastic member, through theplastic member, to an exterior face of a shoulder on the second surfaceof the plastic member, wherein each of said shoulders annularly encircleand support said insert; an electrically conductive, substantiallycompletely hydraulically impermeable second cover resistant to thecorrosive effects of electrolyte matingly contacted with the secondsurface of said plastic member and adapted to minimize contact betweenthe electrolyte and said plastic member within the cell; an electricallyconductive, substantially completely hydraulically impermeable firstcover resistant to the corrosive effects of electrolyte matinglycontacted with the first surface of said plastic member and adapted tominimize contact between the electrolyte and said plastic member withinthe cell; a hydraulically permeable, electrically conductive, currentcollector adjacent to and contacting the first cover; an ion exchangemembrane adjacent to and contacting the current collector; and anelectrically conductive, electrocatalytic material bonded to or embeddedin the surface of the ion exchange membrane contacting the currentcollector.
 2. The frame of claim 1 wherein the second cover is a metalselected from the group consisting of titanium, tantalum, zirconium,tungsten, and alloys thereof.
 3. The frame of claim 1 wherein the firstcover is a metal selected from the group consisting of iron, steel,stainless steel, nickel, lead, molybdenum, cobalt, and alloys thereof.4. The frame of claim 1 wherein the insert is a metal selected from thegroup consisting of aluminum, copper, iron, steel, nickel, titanium, andalloys thereof.
 5. The frame of claim: 1 wherein said second and firstcovers are attached directly to said insert.
 6. The frame of claim 1wherein the second cover is titanium, or an alloy thereof;at least aportion of said inserts are composed of a ferrous-containing material;and said cover is attached, by welding, to at least a portion of saidferrous-containing inserts through an intermediate metal which isweldable compatible with said titanium cover and said ferrous-containinginsert.
 7. The frame of claim 1 wherein the plastic is selected from thegroup consisting of polyethylene, polyproplylene, polyvinylchloride,polystyrene, polysulfone, styrene acrylonitrile, chlorinatepolvinylchloride, acrylonitrile, butadiene and styrene copolymers,epoxy, vinyl esters, polyesters, and fluoroplastics.
 8. The frame ofclaim 1 wherein the plastic contains an additive selected from the groupconsisting of fiberglass, graphite fibers, talc, glass beads, asbestos,and pulverized mica.
 9. The frame of claim 8 wherein the plasticcontains from about 5 to about 75 weight percent of the additive. 10.The frame of claim 1 wherein the plastic contains an additive to reducethermally induced expansion of said plastic member.
 11. The frame ofclaim 1 wherein said plastic member has a peripheral flange extendingoutwardly from the second and the first surfaces of said plastic member.12. The frame of claim 11 wherein the flange extends outwardly from theplastic member about the same distance as said insert.
 13. Anelectrolytic cell comprising:a plurality of the structural frames ofclaim 1 removably and sealably positioned in a generally coplanarrelationship with each other and each of said plastic members beingspaced apart by a second electrode on one side of each of said framesand a first electrode on an opposing side of each of said frames. 14.The cell of claim 13 wherein each of said second and first covers arewelded to at least a portion of said inserts the second electrode andthe first electrode are welded to the respective covers at locationsadjacent to said inserts.
 15. The cell of claim 14 wherein the plasticis selected from the group consisting of polyethylene, polypropylene,polyvinylchloride, chlorinated polyvinylchloride, acrylonitrile,polystyrene, polysulfone, styrene acrylonitrile, butadiene and styrenecopolymers, epoxy, vinyl esters, polyesters, and fluoroplastics.
 16. Thecell of claim 14 wherein the plastic contains an additive selected fromthe group consisting of fiberglass, graphite fibers, carbon fibers,talc, glass beads, asbestos, and pulverized mica.
 17. The cell of claim16 wherein the plastic contains from about 5 to about 75 weight percentof the additive.
 18. The cell of claim 16 wherein said plastic memberhas a peripheral flange extending outwardly from the second and thefirst surfaces of the plastic member.
 19. The cell of claim 13 whereinthe second cover is selected from the group of materials consisting oftitanium, tantalum, zirconium, and tungsten.
 20. The cell of claim 13wherein the first cover is selected from the group of materialsconsisting of iron, steel, stainless steel, nickel, lead, molybdenum,cobalt, and alloys thereof.
 21. The cell of claim 13 wherein the insertis selected from the group of materials consisting of aluminum, copper,iron, steel, nickel, titanium, and alloys thereof.
 22. The cell of claim13 wherein said second and first covers are welded directly to saidinsert.
 23. The cell of claim 13 wherein the second cover is titanium oran alloy thereof, said insert is a ferrous-containing material and avanadium disk is positioned between, and welded to, said second coverand said insert.
 24. The cell of claim 13 wherein the plastic containsan additive to reduce thermally induced expansion of said plasticmember.
 25. The cell of claim 13 wherein said plastic member has aperipheral flange extending outwardly from the second and the firstsurfaces of said plastic member.
 26. The cell of claim 25 wherein theflange extends outwardly from the plastic member about the same distanceas said inserts.
 27. A structural frame adapted for use in achlor-alkali electrolytic cell for producing gaseous chlorine and anaqueous alkali metal hydroxide solution from an aqueous alkali metalchloride brine comprising:a glass filled polypropylene member with aplurality of horizontally and vertically spaced-apart shouldersprotruding outwardly from opposing generally coplanar second and firstsurfaces of said polypropylene member; at least one electricallyconductive steel insert extending through said polypropylene member suchthat said insert is generally annularly encircled by a shoulder on eachopposing surfaces of said polypropylene member; a substantiallycompletely hydraulically impermeable titanium second cover matinglyaffixed to the second surface of said polypropylene member and adaptedto minimize contact between the electrolyte and said polypropylenemember within the cell; and a substantially completely hydraulicallyimpermeable nickel first cover matingly affixed to the first surface ofsaid polypropylene member and adapted to minimize contact between theelectrolyte and said polypropylene within the cell.